To commemorate the milestone of the discovery of the 5000th exoplanet (planet orbiting a star other than the Sun), NASA/JPL produced this video which shows discoveries of exoplanets discovered between 1991 and early 2022 plotted on an all-sky map with the plane of the Milky Way at the equator. Each discovery is shown as a circle whose diameter indicates the size of the planet’s orbit and colour the method of discovery. A musical note announces each exoplanet, with pitch based upon the planet’s orbital period: bass for long periods, treble for short periods.
Note the explosion in discoveries starting in 2011 in one specific area of the sky by the Kepler spacecraft, which stared at that region, monitoring more than 150,000 stars in that patch of sky for dips in brightness caused by transiting planets.
Exoplanet discoveries are not, as you might expect, concentrated along the plane of the Milky Way. This is because the techniques used to find exoplanets work only on relatively nearby stars, which are distributed more or less isotropically in the Sun’s neighbourhood.
Think about something like Jupiter’s Great Red Spot. Suppose a star had an analogous persistent sunspot. (A sunspot is very different from a Great Red Spot – the analogy is to the persistence). Then a slowly rotating star might demonstrate a regular dip in light emission, which could be misinterpreted as a planet passing between the star and the observer.
The presence of multiple planets around our own Sun is highly suggestive that planet formation is a common occurrence in this universe. But are we really sure that every dip in light output is due to a planet transiting the face of its sun?
Starspots are regularly detected by transit observation projects such as Kepler and Tess, and discriminating them from planets is part of the process of declaring detection of an exoplanet. Starspots are not, however, known to persist over long periods of time, and even over short periods they evolve and produce different amounts of dimming as opposed to the absolutely consistent dimming due to a planetary transit. For any detection, two or more transits must be detected, the ingress and outgress curves must be symmetrical, and the depth of transits must be nearly identical and the shape of the ingress and outgress consistent with the computed orbital period. The chance of a starspot mimicking all of these properties is considered very low. Also, the duration of a transit is much less than the rotation period of almost all stars, which would set the duration of a dip due to a starspot.
The Wikipedia section on “Transit photometry” discusses the many causes of false positives and how they are distinguished from genuine exoplanet transits.
